Patent GB-1232224-A (GB)
IPC: H03K17/60
Filed: 1968-11-01
Abstract: 1,232,224. Converting D.C. to A.C. INTERNATIONAL BUSINESS MACHINES CORP. 1 Nov., 1968 [26 Dec., 1967], No. 51906/68. Heading H2F. [Also in Division H3] Turn-on signals are applied to a transistor through one portion of a transformer from a two transistor drive circuit and turn off signals are applied to another portion from a further transistor in response to the cessation of the " on " signals. In Fig. 1, each transistor 20a, 20b of a bipolar switch 20 is turned on and off, in accordance with the invention, during respective half-cycles of a control wave E is applied via transformers 1 and 9. When this wave is such that the upper terminal 11a<SP>1</SP> of transformer 9 is positive a capacitor 23 quickly changes to render a transistor 27 momentarily conductive so as to discharge the " on " period timing capacitor 35. Transistor 37 is thus off and 44 on, thereby completing a path from transformer terminal 12a<SP>1</SP> through the upper half of the primary winding to produce in secondary winding 12c an " on " drive for transistor 20a in unit 20. During this time capacitor 35 is recharging and after a period transistor 37 is thus rendered conductive and 44 cut off. The " on " drive pulse thereupon is terminated, except for a small part which continues to flow from winding 12a through diode 14 into the base of transistor 53 and into the " off period" timing capacitor 60. Transistor 53 thus becomes conductive providing a path through diode 15 to the lower winding 12b of the driver transformer, thus initiating a turn-off pulse. This continues until capacitor 60 has sufficiently charged to break down a Zener diode 64 and thereby to render 67 conductive, cut-off 53 and so terminate the "off" pulse. When E in  subsequently reverses the lower output transistor 20b is likewise driven by "on" and "off" pulses. Diodes 14-17 isolate the transformer primary windings when they are not required to carry current. Inputs V and V 1  are connected in turn, on first switching on, to effect correct starting conditions and a resistor 20i across the input of transistors 20a, 20b may be omitted if the circuit is rearranged so that both transistors are on together. In Fig. 3 (not shown) transistor 67 is omitted and " off drive " timing is effected by a capacitor (60) connected in series with resistor 71 and the base of transistor 53 so that the latter conducts until the capacitor is charged. Fig. 4 (not shown) is a single ended circuit otherwise similar to those described except that instead of providing the control wave E in the primary of the driven transformers 12, 13 a D.C. voltage may be applied at this point and the control wave applied to the capacitor equivalent is 23 and 24 via a selector switch. In Fig. 7 the " off " pulse is initiated when the " on " driven transistor 44<SP>1</SP> is switched off and continues until a capacitor 60<SP>1</SP> has charged sufficiently to make 103 conduct and switch off the off driven transistor 53<SP>1</SP>. When this goes off it switches on a further transistor 104 connected to an additional winding it passes a current (shown at TRB) to provide the desired mean value of current during the on-off cycle.